I'm not sure what JD dealer told you is true but I use Castrol GTX 5/30 in car & 10/30 in Honda mower & honda pressure washer & JD LT with no problems at all. Last season I did however switch the LT to 5/30 Mobil 1 after 22 hrs. You should change synthetic same as reg oil.... the only advantage with synthetic is it will handle the heat better. I would think that JD oil is the same as one of the other oils on the market only with the JD label on it. By the way I've used Castrol oil in all my cars since 1958 with no problems.

your dealer is wrong. as long as you use the correct weight oil and change it at the correct interval(along with the filter), then you will be fine. they CANNOT deny you a warranty claim for not using their oil either, so don't let him tell you he can.

i put Vavloline in my X500 after i changed the break in oil. not supposed to have to change it again until i reach 100 hours, but i WILL change it before then! though in all honestly 100 hours is abotu my annual usage i just don't like runnign oil that long of a time frame.

david is very wise not letting oil change intervals go 100 hours. for some of us that maybe two or three years if you got a smallish yard. (under 2 acres). Everybody that's been here a while knows I'm a oil changing nerd. 50 hours is too long for me. I usually change mind 3x a year requardless of hours. Once it reaches the black gooey stage IMO it's time to come out. The engine manufacture has an SAE oil standard to use. As long as the brand you buy meets that standard it will be OK syn, semi-syn, or not.

I've used synthetic oil in my tractor for 15 years. I change it every fall. It barely looks dirty. From what I understand the synthetic helps the engine run cooler as well as the superior lubrication.
Anyway, whatever oil you use, you want a good oil with plenty of additives. A good oil is all you need. Check the service designation on the can. The newer numbers doesn't necessarily means it's any better, just that they've payed for the testing. JD just puts their name on somebodies oil. It's probably as good as any other oil but probably more expensive.

My John Deere cataloge has 0w40 JD synthetic and says you can add 50% to the drain interval if you also use a JD filter. Also notes potential fuel savings up to 2.7%. Went an fetched my Spring 2009 book just to make certain I was quoting the numbers right.

I use Mobil One synthetic 5w30 for everything in my garage. In my truck I run it 10k between changes.... dark but it still has good body when I change it. Motors turn over much better in cold weather with synthetic and the oil pressure comes up quick.

Seems like there are a thousand ideas about motor oil, and many of them work fine for the person who believes the idea. One rule of thumb we might look at---An "over-the-road-truck" might well average about 50 MPH day in and day out on the highway. So, 100 hours after an oil chage would put the service on the oil at perhaps 5 thousand miles. If every trucking company changed oil at 5 K, they would go broke. Instead they go 20,000 or on up to 30 K depending on the haul and load. Truth is, oils have improved tremendousely. If you are using a name brand synthetic, like Delvac 5W/40 or or Rotella synthetic or such, you can be assured that 100 hours is not overtaxing that oil even if it does take 2 or 3 years to get to that point. If you throw out expensive sythetic oil 2 or 3 times a summer just because the color changed, you may as well throw out some 20 dollar bills too.
The John Deere oil is totally loaded with additives, a lot of additive. The additive pack is what is cleaning and protecting the parts, other quality oil is ditto likewise. If there were no additives, there would be sludge.
I just bought a new 6330 JD. I'm seriousely considering the JD syn oil, although I am a Chevron Delo man and sell Delo. But either will work great.